A basic practical question in treating children with minimal brain dysfunction (MBD) is what drug, if any, for what child. This research will investigate the effect of methylphenidate on frustration (frustrative reward and nonreward) and on one of the best laboratory measures of attentional processes, reaction time. The study will be a double-bind, cross-over design of 6 weeks duration. Each MBD child will undergo a predrug evaluation and an on-drug and on-placebo evaluation. Drug dosage will be carefully regulated according to a predetermined procedure. Side effects will be recorded weekly and rated for severity. Clinical subjects will be 80 Caucasian boys, aged 7 to 10, with Full Scale WISC IQs 100 or above. Ten matched normals will be studied in the control condition. Teacher and parent rating forms and a battery of tests of cognitive style will be employed pre-drug and at 3 and 6 weeks, as will the laboratory experiment. The laboratory run will consist of two phases: a) control study of reaction time to 105 tones (5 intensity levels) from very soft to very loud; and b) a repetition of the tones comparing two conditions of reward and frustration. Dependent variables are the test scores and reaction times and changes in skin resistance, heart rate and evoked responses of the brain. Of major interest are speed of the motor response to tones, practice and fatique effects, interactional effects of stimuli (strong tones on a subsequent weak tone and the opposite), and the effects of reinforcement in sustaining attention. This research is modeled after Russian experiments in which reaction time differentiated between strong and weak nervous systems and were sensitive to the effect of caffeine.